Recently, attention is paid to a sensor network as an important technology in the field of information communication technology. In order to enable a sensor network to pervade society, it is necessary to resolve various issues in relation to systematization technology and hardware technology. Among those issues, an ultra low power consumption integrated circuit technology corresponds to one of technologies having the key to put sensor network to practical use. In order to maintain monitor sensors of a structure or a communication environment of a large number of constructed networks, an enormous labor force is necessary. Therefore, it is required to realize a maintenance free monitoring sensor, and an integrated circuit having a function of processing and transferring sensing information requires a property of ultra low power consumption of a microwatt class, which enables the integrated circuit to operate for at least 10 years using a small-sized battery or natural energy (see Non Patent Literature 1 below).
FIG. 5 illustrates an example of the configuration of a conventional sensor apparatus. The conventional sensor apparatus includes an A/D converter for converting an analog signal from a sensor to a digital signal, an encoder for encoding the digital signal, a modulator for modulating the encoded digital signal, an amplifier for amplifying the modulated signal and transmitting the amplified signal through an antenna, and a wakeup receiver for receiving a signal from the outside through the antenna. Upon receiving a command signal from an external device, such as a data collecting device, the conventional sensor apparatus starts to perform a monitoring function. Among the elements of the sensor apparatus, only the wakeup receiver constantly operates to monitor a command signal. The other elements of the sensor apparatus are normally in an off state and are turned on by a signal from the wakeup receiver. The conventional sensor apparatus as described above can achieve low power consumption to some degree.
FIG. 6 illustrates an example of the configuration of a wireless receiver used as the wakeup receiver constituting the conventional sensor apparatus described above. In the coherent type wireless receiver 901 illustrated in FIG. 6, a phase-modulated or an amplitude-modulated signal is received by an antenna 902, the received signal is amplified by a low noise amplifier 903, and the amplified signal is then mixed with an output of a voltage controlled oscillator 904 having been adjusted to oscillate with the same frequency as a carrier, so that the amplified signal is converted to a baseband signal. Further, a clock data recovery circuit 906 reproduces a clock from the baseband signal amplified by the amplifier 905 and reproduces digital data while adjusting the timing according to this clock.